1 Prof. IHAB YOUNIS BASIC IMMUNOLOGY. 2 The Immune System.

1Prof. IHAB YOUNIS

BASICIMMUNOLOGY

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The Immune System

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Immunityimmunitas(L) = exemption from civic duties & prosecution

Means protection from infectious diseases

2 Levels

Natural Adaptive

Physical barriers

e.g. skin, cillia

Physiological factors e.g. pH

Protein secretions e.g. lysozymes

Phagocytes e.g.PML

The foreign agent is recognised in a specific manner and the immune system acquires memory towards it

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Types of adaptive immunity

I- Humoral immunity

II- Cell mediated immunity

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I- Humoral immunity

1-Activation phase:• Macrophages engulf invader and digest it

by lysozymes• Some of the digested pieces are displayed

on the surface of macrophages so that other cells of the immune system can recognize them as an antigen

• Helper T cells bind to these antigen and become activated

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2- Effector phase:

• Activated T helper cells trigger B cell to proliferate and release antibodies

• These antbodies then bind to the invader and fight infection

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II- Cell mediated immunity

1-Activation phase:• T cytotoxic cells(Tc) that have the

appropriate receptors are activated and triggered to divide

2- Effector phase:• Tc kill infected or cancerous cells by

releasing perforins (a cytolytic protein)into the targeted cell that kill these cells by causeing perforation of the cell wall

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Cells & tissues of the immune system

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I – Cells

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1-LymphocytesI-B lymphocytesII-T lymphocytes

OriginBone marrowArise in bone marrow but mature in the thymus

FunctionProduce antibodies

CD4(T-helper): Activate B cells and other immune cells

CD8(T-cytotoxic):

-Help rid the body of infected & cancer cells

-Responsible for tissue rejection

•CD stands for cluster of differentiation•It is a glycoprotein that is expressed on the surface of T lymphocytes

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III- Natural killer (NK) cells (2% of lymphocytes):

• They are large granular lymphocytes that are cytotoxic in the absence of prior stimulation

• They possess receptors which allow them to detect some infected host cells, including tumor cells, virus, or intracellular bacteria-infected cells and kill them directly by cytokines

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T Cell Development• All T cells originate from hematopoietic stem

cells in the bone marrow• They travel to the thymus and expand by cell

division to generate a large population of immature thymocytes

• About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, while the other 2% survive and leave the thymus to become mature immunocompetent T cells

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B Cells

• B cells are produced in the bone marrow

• During the first infection, the responding

naїve cells (ones which have never been exposed to the antigen) proliferate to produce a colony of cells

• Most of these cells

transform into the plasma

cells which produce the

antibodies

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• The rest persist as the memory cells that can survive for years, or even a lifetime

• On the next exposure to the antigen the number of different clones responding to the same antigen increase and a greater number of memory cells persist. Thus, a stronger response occurs. This is the principle behind vaccination and administration of booster doses

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2-MONONUCLEAR PHAGOCYTES

• These are the cells of the Reticulo-Endothelial System including macrophages, microglial cells in the CNS, endothelial cells of vascular sinusoids and reticular cells of lymphoid organs

T cell

Macrophage

RBC

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They have the following properties:

1. They express a receptor (CD14) which serves as a recognition molecule for a wide variety of bacteria. Ligation of this receptor leads to macrophage activation

2. They can act as antigen presenting cells (APC) for T cells

3. They are activated by T cell derived cytokines leading to increased phagocytosis

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4. They express receptors for antibody and complement which means that they bind immune complexes

5. They act as scavengers for cell debris and senescent cells (e.g. Kupffer cells in the liver bind "old" erythrocytes)

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3 -DENDRITIC CELLS• They are bone marrow derived

• In the skin they are known as Langerhans Cells

• They pick up antigen in skin, process it and carry it to lymph nodes

• In lymph nodes

dendritic cells, may

efficiently present

antigen if they meet

the right T cell

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4- GRANULOCYTES 3 types distinguished according to their

histological staining

• Neutrophils(polymophonuclear leukocytes): express receptors for immunoglobulin and complement and are involved in the acute inflammatory response

• Eosinophils : carry receptors for IgE, are involved in the destruction of IgE coated parasites, such as helminths, and contribute to the response to allergens

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• Basophils : express high affinity receptors for IgE and are stimulated to secrete the chemicals responsible for immediate hypersensitivity following antigen induced aggregation of these receptors

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II-Lymphoid tissues

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Classified as:

• Central lymphoid tissue (bone marrow, thymus)

• Peripheral lymphoid tissue (lymph nodes, spleen, mucosa-associated lymphoid tissue e.g. intestinal Peyer's Patches)

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Antigen-presenting cells(APC)

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Types I-Professional APCs• They are very efficient at internalizing

antigen and then display a fragment of the antigen, bound to a class II MHC molecule, on their membrane. The T cell recognizes and interacts with the antigen-class II MHC molecule complex on the membrane of the antigen presenting cell. An additional co-stimulatory signal is then produced by the antigen presenting cell, leading to activation of the T cell

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• There are three main types of professional antigen-presenting cells:

- Dendritic cells

- Macrophages

- B-cells

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II- Non-professional APC• A non-professional APC does not

constitutively express the MHC proteins required for interaction with naive T cells; these are only expressed upon stimulation of the non-professional APC by certain cytokines such as IFN-γ

• Non-professional APCs include: - Fibroblasts (skin) - Thymic epithelial cells - Thyroid epithelial cells - Glial cells (brain) - Pancreatic beta cells - Vascular endothelial cells

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• What is an antigen? It is any substance that can be bound by an antibody.So, it can be anything from simple chemicals, e.g. sugars, to complex protein e.g. viruses

• What is a hapten? It is a small antigen that is not immunogenic in itself and needs to be coupled to a carrier to elicit an immune response

• ِ�What is an epitope? It is the small unique part of the antigen that is recognized by an antibody

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Antibodies

• They are Y-shaped proteins that are found in blood or other bodily fluids, and are used by the immune system to identify and neutralize foreign objects

• Antibodies belong to a family of large protein molecules known as immunoglobulins

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The sections that make up the tips of the Y's arms vary greatly from one antibody to another. It is these unique contours in the antigen-binding site that allow the antibody to recognize a matching antigen, much as a

lock matches a key

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Antibodies occur in two forms:• A soluble form secreted into the blood and

tissue fluids, and a membrane-bound form attached to the surface of a B cell that is called the B cell receptor (BCR)

• The BCR allows a B cell to detect when a specific antigen is present in the body and triggers B cell activation

• Activated B cells differentiate into either antibody generating plasma cells that secrete soluble antibody, or into memory cells that survive in the body for years

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Types of antibodies

TypeDescription

Ig AFound in mucosal areas, such as the gut, respiratory tract and progenitor tract, and prevents their colonization by pathogens

Ig DFunctions mainly as an antigen receptor on B cells

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Ig EBinds to allergens and triggers histamine release from mast cells, and is involved in allergy. Also protects against parasitic worms

Ig GProvides the majority of antibody-based immunity against invading pathogens

Ig MExpressed on the surface of B cells. Eliminates pathogens in the early stages of B cell mediated immunity before there is sufficient IgG

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Fab and Fc Regions of antibody

• The tip of the Y contains the site that binds antigen and, therefore, recognizes specific foreign objects. This region of the antibody is called the Fab (fragment, antigen binding) region

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The base of the Y is called theFc (Fragment,

crystallizable)

region. The Fc

region binds to

various cell

receptors.By doing

this, it mediates

different physio-

logical effects e.g.

opsonization, cell lysis

Receptor

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Modes of action of antibodies

• Neutralisation: blocking the biological activity of their target molecule e.g a toxin binding to its receptor

• Opsonisation: interacting with special receptors on various cells, including macrophages, neutrophils, basophils and mast cells allowing them to "recognise" and respond to the antigen

• Complement Activation: causes direct lysis by complement-complement recruitment also enhancing phagocytosis

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Monoclonal antibodies

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Production• If an antigen is injected in the body, B-cells

will turn into plasma cells and start to produce antibodies that recognize that antigen. Each B-cell produces only one kind of antibody, but different B-cells will produce structurally different antibodies that bind to different parts (epitopes) of the antigen. This natural mixture of antibodies found in serum is known as polyclonal antibodies

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• To produce monoclonal antibodies, the B-cells from the spleen or lymph nodes are removed from an animal that has been challenged several times with the antigen of interest. These B-cells are then fused with myeloma tumor cells that can grow indefinitely in culture and that have lost the ability to produce antibodies

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• The fused hybrid cells

(called hybridomas),

being cancer cells, will

multiply rapidly and

indefinitely. Large

amounts of antibodies

can therefore be

produced

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Characters of monoclonal antibodies

• They are identical because they were produced by one type of immune cell and are all clones(copies)of a single parent cell

• It is possible to create monoclonal antibodies from any substance so that the produced antibodies can specifically bind to that substance; they can then serve to detect or purify that substance

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Uses of monoclonal antibodies

1-Diagnostic uses:

• Detection of small amounts of drugs, toxins or hormones, e.g. detection of HCG in pregnancy test kits and ELISA test

• Can be used to classify strains of a single pathogen, e.g. Neisseria gonorrhoeae

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2-Therapeutic uses:

• Suppression of the immune system to prevent transplanted organs rejection

• Treatment of some inflammatory diseases e.g. Infliximab (Remicade®) in psoriasis

• Cancer treatment e.g. Rituximab (Rituxan®) in lymphoma

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Complement (NOT compliment)

• The complement system consists of a series of proteins that work to "complement" the work of antibodies in destroying bacteria

• Complement proteins circulate in the blood in an inactive form

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• The "complement cascade" is set off when the first complement molecule, C1, encounters antibody bound to antigen in an antigen-antibody complex. Each of the complement proteins performs its specialized job in turn, acting on the molecule next in line. The end product is a cylinder that punctures the cell membrane and, by allowing fluids and molecules to flow in and out, the target cell is killed

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• The above mentioned sequence of activation of complement is called the classical pathway

• The alternative sequence starts with the C3 component, not C1, thereafter, the sequence follows that of the classical pathway

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Cytokines• They are proteins which mediate and

regulate immunity, inflammation, and hematopoiesis

• They act by binding to specific membrane receptors

• Responses to cytokines include increasing or decreasing expression of membrane proteins (including cytokine receptors), proliferation, and secretion of effector molecules

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Cytokine is a general name; other names include

• Lymphokine : cytokines made by lymphocytes

• Monokine : cytokines made by monocytes

• Chemokine : cytokines with chemotactic activities

• Interleukin : cytokines made by one leukocyte and acting on other leukocytes

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Properties of cytokines

• Redundancy: most functions of cytokines can be performed by many different cytokines

• Pleiotropism : a single cytokine has many different functional effects, litterally on many different cell types but in fact sometimes even on the same cell

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Cytokine Activities

• Cytokines are made by many cell populations, but the predominant producers are helper T cells (Th) and macrophages

• The largest group of cytokines stimulates immune cell proliferation and differentiation

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• The practical manifestation of these properties means, for instance, that blocking a particular cytokine rarely has widespread or dramatic effects , and conversely that overexpression or exogenous administration of a single cytokine frequently has several diverse effects

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CytokineFunction

IL-1activates T cells

IL-2stimulates proliferation of antigen-activated T and B cells

IL-4, IL-5 & IL-6 stimulate proliferation and differentiation of B cells

IL-3, IL-7&GM-CSF (Granulocyte Monocyte Colony-Stimulating Factor)

stimulate hematopoiesis

TNFa TNFbtumor cell death

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Other groups of cytokines include:

• Interferons: IFNα and IFNβ inhibit virus replication in infected cells, while IFNγ also stimulates antigen-presenting cell MHC expression

• Chemokines attract leukocytes to infection sites

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Major Histocompatibility Complex (MHC)

• MHC molecules are membrane bound proteins found on the majority of tissue in a healthy human

• Two classes of MHC exist:

- MHC I : found on almost all tissues

- MHC II : found only on antigen

presenting cells(macrophages, dendritic

cells and B cells)

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• MHC I molecules process proteins present inside the cell and present them on their surface

• MHC II molecules present antigens taken from the surrounding environment, most often foreign cell, and present them to the immune system.

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• MHC I molecules bind to peptides that originate in the cytoplasm and move them to the cell membrane

• In this way, the MHC I molecules serve as flags indicating what type of protein processing is going on inside the cell:

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- If the cell is functioning normally, the MHC I molecules will contain peptides derived from normal cell proteins. The immune system will recognize these proteins as self and presume that all is normal with the cell

- In contrast, if the cell is infected by an intracellular parasite, such as virus, some of the MHC I molecules will contain peptide fragments from the virus. These molecules will then interact with T cells of the immune system and initiate a response

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• MHC II molecules bind to antigens that originate from an exogenous source e.g. leftovers of a killed microorganism and move them to the cell surface. The MHC II molecule, together with the antigen, is then recognized by T cells of the immune system

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• Each class of MHC is represented by more than one locus, in man these are called HLA for Human Leucocyte Antigen. The class I loci are HLA-A,-B and -C and the class II loci HLA-DR, -DQ and -DP group of genes resides on chromosome 6

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How Does the Immune System Normally Keep Us

Healthy?

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• Invading organisms are engulfed by an antigen presenting cell, and their component proteins (antigens) are cut into pieces and displayed on the cell's surface

• Pieces of the antigen bind to the major histocompatibility complex (MHC) proteins, also known as human leukocyte antigen (HLA) molecules, on the surface of the APCs

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• This complex, formed by a foreign protein and an MHC protein, then binds to a receptor on the surface of the CD4 helper T cell

• The antigen-specific CD4 helper T cells multiply while secreting cytokines

• The particular cytokines secreted by the CD4 helper T cells act on the CD8 "cytotoxic" T cells

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• The helper T cells can also activate antigen-specific B cells to produce antibodies, which can neutralize and help eliminate organisms from the body

• Some of the antigen-specific T and B cells that are activated to rid the body of infectious organisms become long-lived "memory" cells. Memory cells have the capacity to act quickly when confronted with the same infectious organism at later times. It is the memory cells that cause us to become "immune" from later reinfections with the same organism

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Hypersensitivity (Allergy)

• It is an immune response to a foreign molecule(allergen), which is damaging rather than helpful to the host

• The first exposure to allergen activates the immune response, but it is the second and subsequent exposures that result in the symptoms we associate with allergies e.g. urticaria, sneezing, coughing

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4 types exist

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Type I Hypersensitivity(Anaphylaxis)

Antigen e.g.pollens, foods(nuts,

sea food,eggs),drugs(penicillin,

aspirin),insect products(bee

venom,house dust mite) is

engulfed by APC that presents

it to TH cells that produce IL-4

that stimulates B cells to be

plasma cell .Plasma cells

produce IgE which binds to

mast cells, basophils &eosinophils

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• A subsequent exposure to the same allergen cross links the cell-bound IgE and triggers mast cell degranulation and release

of various pharmacologically active substances e.g. histamine and serotonin

• Mast cell activation also leads to the production of two other types of

mediators )secondary mediators(i.e. arachidonic acid metabolites (prostaglandins andleukotrienes) and proteins (cytokines and enzymes)

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• The actions of mediators is rapid taking 1-5 minutes to induce symptoms

• The clinical picture depends on which organ systems are affected, as follows:

1-Urticaria/angioedema: if mediators were released in the superficial layers of the skin

2-Allergic rhinitis: if mediators were released in the upper respiratory tract, they can result in sneezing, nasal congestion, rhinorrhea, and itchy or watery eyes

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3-Allergic asthma: if mediators were released in the lower respiratory tract they can cause bronchoconstriction, mucus production, and inflammation of the airways, resulting in chest tightness, dyspnea, and wheezing

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4-Anaphylaxis: if mediators were released in more than one system the reaction is known as anaphylaxis. In addition to the foregoing, the GIT can also be affected with symptoms of nausea, abdominal cramping, and diarrhea. Systemic vasodilatation and vasopermeability can result in significant hypotension and is referred to as anaphylactic shock. This can be life threatening

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Type II Hypersensitivity(cytotoxic hypersensitivity)

• It is caused by IgM or IgG antibodies binding to cells or tissue antigens. The antibodies cause cell destruction either directly or by recruiting complement

• Examples of

this type are

ABO inco-

mpatibility

reaction and

autoimmune diseases

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Type III Hypersensitivityimmune complex hypersensitivity((

• It is now thought that this form of hypersensitivity has a lot in common with type I except that the antibody involved is IgG and therefore not prebound to mast cells, so that only preformed complexes can bind to the low affinity Fc gammaRIII

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• It is mediated by soluble

immune complexes i.e.

the antigen is soluble

and not attached to the

organ involved

• 2 types exist:

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1- Local reaction (The Arthus reaction):• It is a skin reaction in which antigen is

injected into the dermis and reacts with IgG antibodies in the extracellular spaces, activating complement and phagocytic cells to produce a local inflammatory response

• The reaction may take 3 - 10 hours after exposure to the antigen and appears as well defined, slightly raised, indurated, erythematous and oedematous nodule

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2- Systemic type (serum sickness):

• Can result from the injection of large quantities of a poorly catabolized foreign antigen. This illness was so named because it frequently followed the administration of therapeutic horse antiserum e.g. serum from horses immunized with snake venoms

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• Serum sickness occurs 7–10 days after the injection of the horse serum which is the time required to mount a primary immune response that switches from IgM to IgG antibody against the foreign antigens in horse serum

• The clinical features of serum sickness are urticaria, chills, fever, rash, arthritis, and sometimes glomerulonephritis

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Type IV Hypersensitivitycell mediated) or (delayed type hypersensitivity

• There are two phases to a cutaneous hypersensitivity response:

1-sensitization phase : during this phase, cutaneous Langerhans' cells take up and process antigen, and migrate to regional lymph nodes, where they activate T cells with the consequent production of memory T cells,

which end up in the dermis

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2-Elicitation phase : further exposure to the sensitizing chemical leads to antigen presentation to memory T cells in the dermis, with release of T-cell cytokines such as IFN-γ and IL-17. This stimulates the keratinocytes to release cytokines such as IL-1, IL-6, TNF-α which enhance the inflammatory response by inducing the migration of monocytes into the lesion and their maturation into macrophages, and by attracting more T cells

Examples : Contact dermatitis, Tuberculin test and granulomatous diseases e.g. TB and leprosy lesions

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Immunotolerance

• It is recognition of "self", the phenomenon by which the immune system normally suppresses immunological responses to cells of the body, ie self-antigens

• In autoimmune diseases such as SLE, this mechanism is faulty, and the body is attacked by its own immune system

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Mechanisms of tolerance

I- Central tolerance (i.e. tolerance occurring to immature lymphocytes in central lymphoid tissues)

II-Peripheral Tolerance(i.e. tolerance occurring to mature lymphocytes in peripheral lymphoid tissues)

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I- Central tolerance

1-T cells:thymocytes have receptors capable of recognizing both self and foreign antigens. If a thymocyte strongly interacts with a self antigen, it will receives signals that kill it (apoptosis) before it can complete its maturation. This process is termed negative selection

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2- B cells: negative selection of В cells eliminates lymphocytes with high-affinity receptors for self antigens. Another mechanism is called receptor editing where they produce a new Ig light chain that associates with the previously expressed Ig heavy chain to produce a new antigen receptor that is no longer specific for the self antigen

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II-Peripheral Tolerance

There are several such mechanisms: 1-Ignorance:

• T cells and B cells are unaware of the presence of their autoantigen

• Some antigens are sequestered from the immune system in locations which are not freely exposed to surveillance. These are termed immunologically privileged sites. Examples of such sites are the eye, CNS and testis

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2-Anergy :• Naїve T lymphocytes need at least two

signals for their proliferation and differentiation into effector cells: signal 1 is an antigen,

and signal 2 is provided by costimulators that are ex- pressed on pro- fessional (APCs) in response to microbes

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• If T lymphocytes with receptors for the self antigens escape negative selection in thymus, they receive signals 1 when they meet a self antigen, but they do not receive the necessary second signals. Signal 1 without adequate signal 2 induces T cell anergy i.e. antigen recognition without activation

• If mature В cells recognize an antigen and do not receive T cell help (because helper T cells are absent or tolerant), the В cells become anergic

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3-Suppression : On encounter with self antigens, some self-reactive T lymphocytes may develop into regulatory cells whose function is to prevent or suppress the activation of other, potentially harmful, self-reactive lymphocytes

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4- Deletion(activation-induced cell death)• Repeated activation of mature T lymphocytes

by self antigen, or recognition of self antigens without second signals, triggers pathways of apoptosis that result in elimination (deletion) of the self-reactive lymphocytes. Repeated activation of T cells leads to the coexpression of a death receptor called Fas and its ligand, Fas ligand (FasL). continuing to respond

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• FasL binds to Fas on the same or on a neighboring cell. This interaction generates signals through the Fas death receptor that culminate in the activation of caspases, cytosolic enzymes that induce apoptosis. Thus, the repeated activation of the T cell triggers an internal death program that prevents the T cell from

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Autoimmunity

• It is the failure of the body to recognize its own constituent parts as "self", which results in an immune response against its own cells and tissues

• Any disease that results from such an aberrant immune response is termed an autoimmune disease

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• They affect 5% to 7% of the population

• The causes of autoimmune diseases are still obscure

• Nearly 79% of autoimmune disease patients in the USA are women

• They tend to appear during or shortly after puberty

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Examples of autoimmune diseases of the skin

AcceptedProbable

Pemphigus gestat.Alopecia universalis

Lupus erythematosusBehçet's disease

PemphigusHidradenitis suppurat.

Reiter's syndromePsoriasis

Sarcoidosis

Vitiligo

Scleroderma